Satellite based positioning of a wireless terminal

ABSTRACT

At least one set of values of parameters is determined, each set of values defining a respective troposphere model. The at least one determined set of parameter values is then assembled for transmission via a wireless communication network to a wireless terminal as assistance data for an assisted satellite based positioning of the wireless terminal.

FIELD OF THE INVENTION

The invention relates to methods for supporting a satellite basedpositioning of a wireless terminal. The invention relates equally to acorresponding apparatus, to a corresponding network element, to acorresponding system, to a corresponding wireless terminal arrangementand to corresponding software program products.

BACKGROUND OF THE INVENTION

A satellite based positioning of a device is supported by various GlobalNavigation Satellite Systems (GNSS). These include for example theAmerican Global Positioning System (GPS), the Russian Global OrbitingNavigation Satellite System (GLONASS), the future European systemGALILEO, the Space Based Augmentation System (SBAS), the Japanese GPSaugmentation Quasi-Zenith Satellite System (QZSS), the Locals AreaAugmentation System (LAAS), and hybrid systems.

It is the general idea of GNSS positioning to receive satellite signalsat a receiver, which is to be positioned, and to determine the time ittook the signals to propagate from a respective satellite to thereceiver. This time of flight can be determined for example based on ameasured time of arrival of the satellite signal at the receiver andbased on information on the time of transmission of the signal at thesatellite, which is included in the received signal. By multiplying thetime of flight with the speed of light, it is converted to the distancebetween the receiver and the respective satellite. Further, thepositions of the satellites at the respective time of transmission areestimated, for example based on information included in the signal.

The computed distances and the estimated positions of the satellitesthen permit a calculation of the current position of the receiver, sincethe receiver is located at an intersection of the distances from a setof satellites.

The assumption that the distance between the receiver and a respectivesatellite can be obtained by multiplying the determined time of flightof a satellite signal with the speed of light is based on asimplification, though. A signal traveling from a satellite to a GNSSreceiver passes through the electrically neutral atmosphere, includingthe troposphere and the stratosphere. This electrically neutralatmosphere comprises different and varying refractive indices atdifferent locations. The variability of the refractive index causes thesatellite signals to be affected by a path delay and by ray bending. Theeffect is commonly known as tropospheric delay or slant delay. It isdifficult to compensate fully for the tropospheric delay. It constitutesone of the major residual error sources in GNSS navigation solutions.

For correcting the delay, it is generally assumed that the atmosphere ishorizontally layered and azimuthally symmetric. The tropospheric delaymay then be determined as a sum of two components, namely a hydrostaticcomponent and a non-hydrostatic component. The hydrostatic component isdue to atmospheric gases that are in hydrostatic equilibrium, includingusually dry gases and part of the water vapor. The non-hydrostaticcomponent is due water vapor in the atmosphere that is not inhydrostatic equilibrium. Moreover, each of these components can beexpressed as the product of the delay experienced by the GNSS signals inthe zenith direction and a mapping function, which models the elevationangle dependency of the tropospheric delay. With such an approach, thetotal tropospheric delay Z(ε) of a particular satellite signal can bedetermined according to the following equation:Z(ε)=Z _(h) ·m _(h)(ε)+Z _(w) ·m _(w)(ε)  (1)

Here, Z_(h) is the hydrostatic zenith delay, that is, the delay due to ahydrostatic influence that would be experienced by a signal travelingfrom a satellite to the receiver, when the satellite is located at thezenith of the receiver. Further, m_(h)(ε) is the hydrostatic mappingfunction for a given elevation angle ε of a satellite at the currentposition of the receiver. Z_(w) is the non-hydrostatic zenith delay,that is, the delay due to a non-hydrostatic influence that would beexperienced by a signal traveling from a satellite to the receiver, whenthe satellite is located at the zenith of the receiver. Further,m_(w)(ε) is the non-hydrostatic mapping function for a given elevationangle ε of a satellite at the current position of the receiver. Asatellite at the zenith of a receiver has an elevation angle of 90°.

There are various known approaches for determining the zenith delay,like the Hopfield model or the SAAS model for the hydrostatic zenithdelay and the Mendes model, the SAAS model, the Ifadis model or theHopfield model for the non-hydrostatic zenith delay. Further, the zenithdelay could be determined using numerical weather model data.

Typical troposphere delay models moreover use mapping functions in theform of continued fractions. The hydrostatic mapping function m_(h) andthe wet mapping function m_(w) for an elevation angle ε may take forexample the following form: $\begin{matrix}{{m_{h/w}(ɛ)} = \frac{1 + \frac{a_{0}}{1 + \frac{a_{1}}{1 + a_{2}}}}{{\sin(ɛ)} + \frac{a_{0}}{{\sin(ɛ)} + \frac{a_{1}}{{\sin(ɛ)} + a_{2}}}}} & (2)\end{matrix}$

For most existing mapping functions, the values of parameters a₀, a₁ anda₂ in equation (2) are determined separately for the hydrostatic delayand for the non-hydrostatic delay based on values of meteorologicalparameters, such as surface pressure and temperature. Examples are theHerring mapping functions, the Niell Mapping Functions (NMF), theIsobaric Mapping Functions (IMF) and the Vienna Mapping Functions (VMF).The Global Mapping Function (GMF) is moreover an empirical mappingfunction that is based on numerical weather model data. The model isdetermined by using a 15×15 degrees grid of monthly mean profiles forpressure, temperature and humidity from the European Centre forMedium-Range Weather Forecasts (ECMWF).

While equations (1) and (2) are based on the assumption that theatmosphere is azimuthally symmetric, pressure, temperature and humiditygradients may cause in addition in horizontal gradients in therefractivity field. This azimuthal asymmetry may introduce significanterrors in measurements where high precision is required. To take accountof this effect, it is possible to distinguish between the azimuthallysymmetric delay, also referred to as isotropic delay, and asymmetricparts of the delay, also referred to as anisotropic delay. While aboveequation (1) for the total tropospheric delay only includes theisotropic delay, it can be supplemented by a third term for theanisotropic delay as follows:Z(ε,φ)=Z _(h) ^(·m) _(h)(ε)+Z _(w) ·m _(w)(ε)+m _(g)(ε)cot ε(z ₀ cos φ+z₁ sin φ)  (3)

In this equation, z₀ and z₁ are path delay gradient parameters, forinstance vertically integrated refractivity gradients, in the North andEast direction, respectively. The parameter φ is the azimuth angle atwhich a satellite is visible at a GNSS receiver, and m_(g) is a specialmapping function for the gradient term. It has to be noted that theanisotropic part of the equation could also be determined separately forhydrostatic and non-hydrostatic delay. A combination does not cause asignificant loss of accuracy, though. Further, it has to be noted thatthe mapping function m_(g) is not critical and can be chosen equal tom_(h) or m_(w).

In many situations, in particular with mobile GNSS receivers,meteorological parameter values are not available and nominal globalparameter sets that are based on long term statistics are used forcorrecting tropospheric delays. Such global troposphere models, however,are not accurate enough for high precision positioning applications.

In some geodetic surveying applications, which are used for bridgeconstructions etc., zenith delay corrections and surface meteorologicalparameters are sent to the user, for example in the RINEX (receiverindependent exchange) format. RINEX atmospheric corrections are mostlyused in post processing, though, not in real time. The transmittedvalues are moreover limited to a particular position and have no abilityto adjust, if the location or the altitude of the user changes. Further,the time dependency of the provided values is not taken into account, asit is assumed that a new parameter set is transmitted at shortintervals, for example every second.

If a GNSS receiver is integrated into a wireless terminal or if a GNSSreceiver is an accessory for a wireless terminal, the GNSS receiver maybe assisted in the positioning by a wireless communication network. Thewireless communication network may provide assistance data, which isreceived by the wireless terminal and used for improving the performanceof the GNSS receiver. In addition, the wireless terminal may providemeasurement results of the GNSS receiver to the wireless communicationnetwork, which performs the required positioning computations.

The availability of assistance data can greatly affect the performanceof a GNSS receiver. The format in which assistance data may be sent to awireless terminal is specified in various wireless communicationstandards. Control Plane solutions include Radio Resource LocationServices Protocol (RRLP) in Global System for Mobile Communications(GSM), Rate Range Correction (RRC) in Wideband Code Division MultipleAccess (W-CDMA) and IS-801.1/IS-801.A in Code Division Multiple Access(CDMA). Broadcast assistance data information elements for GSM aredefined in 3GPP Technical Specification 44.035, V6.0.0: “Broadcastnetwork assistance for Enhanced Observed Time Difference (E-OTD) andGlobal Positioning System (GPS) positioning methods”. Finally, there areUser Plane solutions OMA SUPL 1.0 by the Open Mobile Alliance, andvarious proprietary solutions for CDMA networks.

The assistance data that is provided according to current wirelessstandards may include for example a reference time, a referencelocation, clock correction data and ephemeris data, which describes asection of a satellite path for a short period of time, etc. However, itdoes not support a GNSS signal propagation delay estimation.

SUMMARY OF THE INVENTION

It is an object of the invention to support high accuracy satellitebased positioning applications in wireless terminals.

A first method is proposed, which comprises determining at least one setof values of parameters, each set of values defining a respectivetroposphere model. The method further comprises assembling the at leastone determined set of parameter values for transmission via a wirelesscommunication network to a wireless terminal as assistance data for anassisted satellite based positioning of the wireless terminal.

Moreover, an apparatus is proposed, which comprises a processingcomponent. The processing component is configured to determine at leastone set of values of parameters, each set of values defining arespective troposphere model. The processing component is furtherconfigured to assemble the at least one determined set of parametervalues for transmission via a wireless communication network to awireless terminal as assistance data for an assisted satellite basedpositioning of the wireless terminal.

The processing component could be realized in hardware and/or software.It could be for instance a processor executing corresponding softwareprogram code, or a chip or chipset having an integrated circuitrealizing the required functions.

Moreover, a network element of a wireless communication network isproposed. The network element comprises the proposed apparatus. Inaddition, it comprises a transmitting component configured to transmitat least one set of parameter values determined and assembled by theapparatus to a wireless terminal as assistance data for an assistedsatellite based positioning of the wireless terminal.

The network element could be for instance a server of an assistanceprovider, which forwards the assembled values to a base station of thewireless communication network for a radio frequency transmission in acell that is served by the base station. Such a transmission by a basestation could be for example a point-to-point transmission to aparticular wireless terminal, a broadcast transmission to any wirelessterminal located sufficiently close to the base station, or a multicasttransmission to any wireless terminal located sufficiently close to thebase station and having subscribed to an assistance service.

Moreover, a system is proposed. The system comprises the proposedapparatus, for example integrated into a network element of a wirelesscommunication network. The system further comprises a wireless terminalconfigured to receive at least one set of parameter values determinedand assembled by the apparatus via a wireless communication network.

Moreover, a first software program product is proposed, in which asoftware program code is stored in a readable medium. When beingexecuted by a processor, the software program code realizes the steps ofthe first proposed method.

For the side of the wireless terminal, further a method is proposed,which comprises receiving at a wireless terminal via a wirelesscommunication network at least one set of values of parameters, each setof values defining a respective troposphere model. The method furthercomprises using an available set of values as assistance data for anassisted satellite based positioning of the wireless terminal.

Moreover, a wireless terminal arrangement is proposed, which comprises amobile communication component configured to receive via a wirelesscommunication network at least one set of values of parameters, each setof values defining a respective troposphere model. The wireless terminalarrangement further comprises a positioning component configured to usean available set of values as assistance data for an assisted satellitebased positioning of the wireless terminal.

The wireless terminal arrangement could be a wireless terminal having anintegrated positioning component. Alternatively, the wireless terminalarrangement could be a combination of a wireless terminal and anaccessory device comprising the positioning component.

Finally, a second software program product is proposed, in which asoftware program code is stored in a readable medium. When beingexecuted by a processor, the software program code realizing the stepsof the second proposed method.

The invention proceeds from the idea that while the current wirelessstandards do not specify any troposphere model that could support theestimation of GNSS signal propagation delay at a wireless terminal, sucha model could be provided as additional GNSS assistance data. It istherefore proposed that parameter values defining a respectivetroposphere model are determined and provided for transmission. Awireless terminal arrangement may receive such a model and use areceived model as assistance data. The assistance data enables in thiscase a determination of a tropospheric delay, which is experienced byreceived satellite signals. This delay may then be considered in thepositioning calculations.

It is an advantage of the invention that it enables a troposphere delayestimation based on flexibly determined troposphere models. This makesthe troposphere delay estimation suitable for high accuracy GNSSnavigation solutions.

In one embodiment of the invention, the parameters include at least oneparameter for defining a time interval during which a troposphere modelis valid. A limitation in time of the validity of a respective modelallows providing models that are more accurate. The parameters fordefining a time interval may comprise for example a parameter fordefining a baseline time and a parameter for defining an interval lengthfollowing upon the baseline time.

In one embodiment of the invention, the parameters include at least oneparameter for defining an area in which a troposphere model is valid.The model is thus localized, not global, which allows as well providingmodels that are more accurate. The parameters for defining a validityarea may comprise for example parameters for defining a central point ofthe area and a parameter for defining a size of the area. For instance,in case the area is specified to be a rectangle, the parameter fordefining a size of the area could be the distance from the center pointof the rectangle to a corner point of the rectangle.

In order to cover an extended time-period and/or a large coverage areawhile maintaining the accuracy of a respective model, severaltroposphere models could be provided, which are valid for differenttimes and/or for different areas.

In general, an assistance provider could determine the required time andarea that should be covered based on a respective use case and arespective application for which a satellite based positioning isrequired.

A wireless terminal arrangement could then select for a respectivepositioning an available set of parameters values of a troposphere modelthat is valid for a current time and for a current location of thewireless terminal as assistance data, and thus take account of spatialand temporal variations of the tropospheric delay, which is not possiblewith the currently employed global troposphere model.

In one embodiment of the invention, the parameters further include atleast one zenith delay parameter for defining a tropospheric delay thatwould be experienced by a signal transmitted by a satellite and receivedby the wireless terminal, when the satellite was located at a zenith ofthe wireless terminal.

Advantageously, a hydrostatic zenith delay parameter and anon-hydrostatic zenith delay parameter are used.

The parameters may further include at least one parameter for specifyingan adaptation rule, which can be used for adapting a value of the atleast one zenith delay parameter to a current altitude of the wirelessterminal. Such an adaptation can be performed for example separately forhydrostatic and non-hydrostatic zenith delay. Such height scale termsallow a zenith delay estimation at different user altitudes and enablethus a further increase of the accuracy of the troposphere model.

In one embodiment of the invention, the parameters further includeparameters for at least one mapping function. Such a mapping functionmaps a value of the zenith delay parameter to a tropospheric delay thatis experienced by a signal transmitted by a satellite and received bythe wireless terminal, when the satellite is located at a knownelevation angle to the wireless terminal. That is, the mapping functionresults in different values for different elevation angles.

Advantageously, a separate mapping function is provided for thehydrostatic and the non-hydrostatic components of the troposphericdelay.

While the mapping functions may be provided for the azimuthallysymmetric distribution of the troposphere delay, the parameters mayfurther include at least two gradient parameters for defining anazimuthally asymmetric effect of the tropospheric delay.

It is to be understood that while the above examples differentiatebetween a hydrostatic and a non-hydrostatic component for zenith delayand mapping function, there could equally be a differentiation between apurely dry component and a wet component. In this case, the water vaporof the hydrostatic component could be considered in the scope of the wetcomponent.

The invention can be employed for any type of wireless communicationsystem, including but not limited to GSM, CDMA and W-CDMA. Further, theinvention can be employed for providing assistance data for any type ofGNSS positioning, including but not limited to GPS and GALILEO.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims. It should be further understood that thedrawings are not drawn to scale and that they are merely intended toconceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic block diagram of a communication system accordingto an embodiment of the invention;

FIG. 2 is a flow chart illustrating a first exemplary operation in thecommunication system of FIG. 1;

FIG. 3 is a table defining message fields for parameters of atroposphere model;

FIG. 4 is a table defining the bits of a fit interval parameter of atroposphere model; and

FIG. 5 is a flow chart illustrating a second exemplary operation in thecommunication system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic block diagram of an exemplary communicationsystem, which supports a satellite based positioning in accordance withan embodiment of the invention.

The system comprises a wireless terminal 100 and a wirelesscommunication network including an assistance server 200. The wirelesscommunication network can be for example a GSM, a CDMA or a W-CDMAnetwork. It has to be noted that the assistance server 200 could also beconnected to the communication network without belonging to thecommunication network. The assistance server 200 is further connected toa reference network 300.

The wireless terminal 100 includes a mobile communication component 110,which may include any component of a conventional wireless terminal likea mobile phone. The mobile communication component 110 enables thewireless terminal 100 to establish a link to a base station of thewireless communication network, which is serving a cell in which thewireless terminal 100 is currently located.

The wireless terminal 100 further includes a GNSS component 120, forexample a GPS or Galileo component. It has to be noted that the GNSScomponent 120 could be realized alternatively as a detachable accessorydevice for a wireless terminal like a mobile phone. The GNSS component120 comprises a GNSS receiver 121 that is connected to a GNSS antenna(not shown). The GNSS receiver 121 is adapted to signals from aplurality of GNSS satellites 400. The GNSS component 120 furthercomprises a processor 122 and a memory 126. The processor 122 is adaptedto execute various installed software program codes. The installedsoftware program codes include at least positioning software programcode 123 and tropospheric delay calculations software program code 124.The memory 126 stores a database 127 for parameter values of a pluralityof troposphere models. The memory 126 could also store the softwareprogram codes 123, 124 for retrieval by the processor 122 when needed.

It is to be understood that the functions of the processor 122 and/orthe memory 126 could alternatively be realized by a processor and amemory, respectively, of the mobile communication component 110. It isfurther to be understood that instead of or in addition to a processor122 executing software program codes 123, 124, a chip with circuitryrealizing corresponding functions could be employed.

The functions illustrated by the processor 122 when executing softwareprogram code 124 can further be viewed as means for receiving via awireless communication network at least one set of values of parameters,each set of values defining a respective troposphere model.Alternatively, the mobile communication component 110 could be viewed asmeans for receiving via a wireless communication network at least oneset of values of parameters, each set of values defining a respectivetroposphere model. The processor 122 when executing software programcodes 123, 124 can be viewed in addition as means for using an availableset of values as assistance data for an assisted satellite basedpositioning of a wireless terminal.

The wireless terminal 100 is an exemplary embodiment of the wirelessterminal arrangement according to the invention.

The assistance server 200 comprises equally a processor 210 and a memory220. The processor 210 is adapted to execute various installed softwareprogram codes. The installed software program codes include a softwareprogram code for updating a database in the memory 211, a softwareprogram code for determining troposphere model parameters 212 and asoftware program code for assembling assistance data 213. The memory 220stores a database 221 with surface meteorological data and/or numericalweather model data. The numerical weather model may be a global model ora local area model, like the High Resolution Limited Area Model(HIRLAM). The memory 220 could also store the software program codes211, 212, 213 for retrieval by the processor 210 when needed. Theassistance server 200 further comprises a receiving and transmittingcomponent (RX/TX) 230 for exchanging data with other elements (notshown) of the wireless communication network.

The assistance server 200 is an exemplary embodiment of the networkelement according to the invention. The processor 210 and theimplemented software program code 211, 212, 213 are an exemplaryprocessing component of an exemplary embodiment of an apparatusaccording to the invention. The apparatus could comprise in additionother components of the assistance server 200, like the memory 220.

The functions illustrated by the processor 210 when executing softwareprogram code 212 can further be viewed as means for determining at leastone set of values of parameters, each set of values defining arespective troposphere model, while the processor 210 when executingsoftware program code 213 can be viewed as means for assembling the atleast one determined set of parameter values for transmission via awireless communication network to a wireless terminal as assistance datafor an assisted satellite based positioning of the wireless terminal.

The reference network 300 comprises a large number of measurementstations at different locations. The measurement stations may be adaptedto receive GNSS signals and/or to perform meteorological measurements.

A first exemplary operation in the system of FIG. 1 will now bedescribed with reference to the flow chart of FIG. 2.

The assistance server 200 constantly receives measurement results fromthe reference network 300.

On the one hand, the assistance server 200 may receive information forconventional assistance data, for example ephemeris data for allsatellites. Based on the received information, the processor 210 of theassistance server 200 may assemble conventional assistance data for arespective base station of the wireless communication network (step501). It may include for example the location of the base station, areference time and ephemeris data for all satellites that are currentlyvisible at the base station. The assistance data can be provided to thebase station for broadcasting in the cell.

On the other hand, the assistance server 200 receives measuredmeteorological data, possibly from all over the world. Such data mayinclude for example air pressure, temperature and humidity profiles.These meteorological data are used by the processor 210 for updating thedatabase 221 in the memory 220. Alternatively or in addition, theassistance server 200 could acquire and process or receive numericalweather model data for updating the database 221 in the memory 220.

The GNSS receiver 121 acquires and tracks satellite signals from aplurality of satellites 400. It determines the time of arrival ofsignals and decodes the included navigation messages. The processor 122of the GNSS component 120 then determines the current position of thewireless terminal 100 based on the information in the navigationmessages and on the time of arrival of the satellite signals (step 511).

In case the mobile communication component 110 receives conventionalassistance data in a broadcast message from the assistance provider 200,it provides the data to the GNSS component 120, where they are used forsupporting the signal acquisition and/or the position calculations.

While a fairly exact position of the wireless terminal 100 can bedetermined this way, the application for which the position is requiredmight need a high precision. In this case, the processor 122 furtherrequests via the mobile communication component 110 a troposphere modelfrom the assistance server 200 (step 512). The request may includelatitude and longitude values of the determined position, C_(lat) andC_(lon).

The assistance server 200 receives the request and the processor 210determines thereupon the parameter values for one or more tropospheremodels, which are valid for a center position defined by the receivedco-ordinates (step 502). The parameters values are determined based onmeasurement results and/or numerical weather model data that are storedin the database 221. The values are inserted into fields of a messagethat will be transmitted to the wireless terminal 100.

FIG. 3 is a table, which presents in a first column the parameters thatdefine a respective troposphere model. The table further presents in asecond column the number of bits of a field that is provided in amessage for the respective parameter. In order to ensure that theparameter values fit into the provided fields, they are scaled by apredetermined amount and, if necessary, rounded to an integer value. Thetable presents in a third column the scale factor by which the integerparameter value has to be scaled at the wireless terminal to regain theoriginal values. The table further presents in a fourth column the unitof the parameter, if applicable.

A first group of parameters comprises troposphere model controlparameters. They are provided once per model.

The troposphere model control parameters include a parameter “UTC”,which indicates the baseline time in seconds for which the parameters ofa model are valid. For this compulsory parameter, a field of 32 bits isprovided. The parameter value is inserted in the provided field withoutscaling.

The troposphere model control parameters further include a parameter“Fit Interval”, which specifies the validity period of the model. Forthis parameter, a field of 6 bits is provided. The parameter can havevalues, which indicate a validity period in a range of 0.125 to 448hours.

The “Fit Interval” parameter is specified according to a specialfloating-point representation as illustrated in FIG. 4. FIG. 4 is atable, which presents in a first column possible values of the firstthree bits of the “Fit Interval” field and in a second column possiblevalues of the last three bits of the “Fit Interval” field. The firstthree bits represent an exponent e, while the last three bits representa mantissa m. The third column indicate a floating-point value x, whichis obtained by a respective combination of an exponent e and a mantissam. The fourth column indicates a Fit Interval value F, which indicates aperiod in hours h that corresponds to a respective floating-point valuex. For example, if the field has a value of ‘110110’, this correspondsto an exponent of e=3 and a mantissa of m=3. This constellation is dealtwith in the fourth row and results in x=(m+1)*2^((e-1))=16, that is, theFit Interval has a value of F=16 hours.

The floating-point value of x=512 has a special meaning; it indicates aninfinite Fit Interval for the troposphere model.

The troposphere model control parameters further include parameters“C_(lat)” and “C_(lon)”, which define the latitude and the longitude insemi-circle notation, respectively, of a central point for which themodel is valid. For these parameters, a respective field of 14 bits isprovided. The parameter values are inserted with a scaling of 2¹³ intothe provided fields. The troposphere model control parameters furtherinclude a parameter “R_(c)”, which indicates the distance from thecenter point to the corner point of a rectangular fit area in meters.For this parameter, a field of 10 bits is provided. The parameter valueis inserted in the provided field with a scaling of 2⁻¹⁰. Therectangular fit area is the area for which the model is valid. Apredefined maximum value of R_(c) indicates that the model is global.

A second group of parameters comprises zenith delay parameters. They areequally provided once per model.

The zenith delay parameters include a parameter “Z_(h0)”, whichindicates the hydrostatic zenith delay in meters, and a parameter“Z_(w0)”, which indicates the non-hydrostatic zenith delay in meters.The parameters are calculated to the WGS-84/EGM96 reference geoid. Forthese parameters, a respective field of 12 bits is provided. Theparameter values are inserted in the provided fields with a scaling of2⁻¹⁰. The parameters can be determined in a conventional manner, forinstance using either surface meteorological measurements,meteorological parameters from a gridded database or numerical weathermodels.

The zenith delay parameters also include height scale terms s_(o), s₁,e_(h) and e_(w) that can be used for computing the respective zenithdelay at different altitudes h of the wireless terminal. For s₀ and s₁ arespective field of 14 bits is provided and the values are inserted inthe provided fields with a scaling of 2²⁶ and 2³², respectively. Fore_(h) and e_(w) a respective field of 12 bits is provided and the valuesare inserted in the provided fields with a scaling of 2²⁰.

A third group of parameters comprise mapping function parameters. Theyare equally provided once per model.

The mapping function parameters include parameters “α₀”, “α₁” and “α₂”,which specify continued fractions mapping function parameters for thehydrostatic delay. The mapping function parameters further includeparameters “β₀”, “β₁” and “β₂”, which specify continued fractionsmapping function parameters for the non-hydrostatic delay. These mappingfunctions are used to map the given zenith delays to a particularelevation angle at which a satellite 400 is positioned with relation tothe wireless terminal 100. For these parameters, a respective field of16 bits is provided. The parameter values are inserted in the providedfields with a scaling of 2¹⁹. The parameters can be determined in aconventional manner.

Finally, the mapping function parameters include parameters “z₀” and“z₁”, which enable an estimation of an azimuthal asymmetry (anisotropy)of the troposphere delay. For these parameters, a respective field of 12bits is provided. The parameter values are inserted in the providedfields with a scaling of 2⁵. The azimuthal asymmetry results frompressure, temperature and humidity gradients. The parameters can thus bedetermined in a conventional manner based on correspondingmeteorological measurements or based on corresponding data fromavailable weather models.

The troposphere model format presented in FIG. 3 enables a long-termlocalized troposphere delay estimation for high accuracy GNSS navigationsolutions. It is suitable for all wireless standards and all GNSSsystems.

It has to be noted that the number of bits and the indicated scalingfactors in the table of FIG. 3 are only provided as examples. They canbe varied in many ways. Further, also the indicated parametersthemselves can be varied, as long as they represent in their entirety atroposphere model. For example, instead of hydrostatic andnon-hydrostatic zenith delay and mapping function parameters, dry andwet zenith delay and mapping function parameters could be used.

The processor 210 of the assistance server 200 could determine forexample parameters for a first, highly accurate model having a small fitarea and a short fit interval. In addition, it could determine forexample parameters for a second, less accurate model having a large fitarea and a long fit interval.

The assistance server 200 then transmits a message with the determinedmodel parameter values to the wireless terminal 100.

In the wireless terminal 100, the mobile communication component 110forwards the received model parameter values to the processor 122, whichstores the values in the database 127 in the memory 126 (step 513).

Further, the processor 122 selects the most suitable of all models, forwhich parameter values are currently stored in the database 127 (step514). The model has to be valid for the current time and at the currentposition of the wireless terminal 100 as determined in step 511. The fitinterval is determined by the processor 122 based on the definitions inthe table of FIG. 4. In case there are several models meeting theseconditions, the one with the smallest fit interval and the shortest fitarea may be selected, in order to obtain the most accurate model.

The processor 122 may moreover delete all parameter values from thedatabase 127 that belong to a model of which the fit interval haspassed. The parameter values that belong to a model of which the fitarea does not fit to the current position of the wireless terminal butof which the fit interval is still valid may be kept in the database 127for the case that the wireless terminal 100 returns to the fit area at alater point of time.

In case a suitable model is available (step 515), the processor 122determines the elevation angle ε and the azimuth φ for all or selectedvisible satellites (step 516). The angles can be determined based on theposition of the wireless terminal 100 and on the position of thesatellites 400, both being known from the positioning in step 511.

The processor 122 then determines the hydrostatic zenith delay Z_(h)(h)and the non-hydrostatic zenith delay Z_(w)(h) at the current altitude ofthe wireless terminal 100 as follows:Z _(h)(h)=s ₀ *h+s ₁ *h ² +Z _(h0)*exp(−e _(h) *h),  (4)Z _(w)(h)=Z _(w0)*exp(−e _(w) *h),  (5)where Z_(h0), Z_(w0), s₀, s₁, e_(h) and e_(w) are the stored zenithdelay parameter values of the selected troposphere model, and where h isthe altitude of the wireless terminal 100 determined in the positioningof step 511, given in meters from WGS-84/EGM96 reference geoid. Whenapplying the equations, the stored zenith-delay parameter values arescaled with the associated scale factor indicated in the table of FIG.3. This zenith delay model could also be used as a reference model forbarometric altimeter.

Further this zenith delay model could be utilized as a reference modelfor a barometric altimeter.

The 122 processor further determines for each of the consideredsatellites 400 the hydrostatic mapping function m_(h) for the determinedelevation angle ε as follows: $\begin{matrix}{{{m_{h}(ɛ)} = \frac{1 + \frac{\alpha_{0}}{1 + \frac{\alpha_{1}}{1 + \alpha_{2}}}}{{\sin(ɛ)} + \frac{\alpha_{0}}{{\sin(ɛ)} + \frac{\alpha_{1}}{{\sin(ɛ)} + \alpha_{2}}}}},} & (6)\end{matrix}$where α₀, α₁ and α₂ are the stored mapping parameter values for thehydrostatic mapping function.

The processor 122 determines in addition for each of the consideredsatellites 400 the non-hydrostatic mapping function m_(h) for thedetermined elevation angle ε as follows: $\begin{matrix}{{{m_{w}(ɛ)} = \frac{1 + \frac{\beta_{0}}{1 + \frac{\beta_{1}}{1 + \beta_{2}}}}{{\sin(ɛ)} + \frac{\beta_{0}}{{\sin(ɛ)} + \frac{\beta_{1}}{{\sin(ɛ)} + \beta_{2}}}}},} & (7)\end{matrix}$where β₀, β₁ and β₂ are the stored mapping parameter values for thenon-hydrostatic mapping function.

When applying the equations for the mapping functions, again all storedmapping parameter values are scaled with the associated scale factorindicated in the table of FIG. 3.

Finally, the processor 122 determines for each of the consideredsatellites 400 the total tropospheric delay Z as follows (step 518):Z(ε,φ,h)=Z _(h)(h)·m _(h)(ε)+Z _(w)(h)·m _(w)(ε)+m _(h)(ε)·cot ε[z ₀ cosφ+z ₁ sin φ]  (8)

When applying this equation, also the parameter values z₀ and z₁ arescaled with the associated scale factor indicated in the table of FIG.3. The first two terms on the right hand side of this equation indicatethe isotropic part of the tropospheric delay, while the third termindicates the anisotropic part, which reflects azimuthal asymmetries inthe horizontal layers of the troposphere. It has to be noted that itwould also be possible to use the non-hydrostatic mapping function m_(w)instead of the hydrostatic mapping function m_(h) in the third term.Further, the assistance server 200 could also provide separate parametervalues for hydrostatic and non-hydrostatic delay, in order to enable theprocessor 122 to determine the third term separately for hydrostatic andnon-hydrostatic delay.

The total tropospheric delay Z is thus adjusted to the altitude of thewireless terminal 100, to the elevation angle of the consideredsatellite 400 and to the azimuth of the considered satellite 400.

The position of the wireless terminal 100 can then be determined againmore accurately by taking into account the total tropospheric delay ofthe signals when determining the progagation time of the signals from aparticular satellite 400 to the wireless terminal 100 (step 519).

The process can be continued with step 514, as long as availabletroposphere models are found in the database 127 in step 514. In case notroposphere model is found in the database 127 that is valid for thecurrent time and the current position of the wireless terminal 100 (step515), the processor 122 may proceed with requesting new tropospheremodels from the assistance server 200 via the mobile communicationcomponent 110 (step 512).

In the operation presented with reference to FIG. 2, the tropospheremodels are provided by the assistance server 200 upon request from awireless terminal 100. It is to be understood, though, that they couldalso be generated at regular intervals for all or selected base stationsof the wireless communication network and broadcast in the cell of arespective base station.

A second exemplary operation in the system of FIG. 1 using suchbroadcast messages will now be described with reference to the flowchart of FIG. 5.

In this case, the processor 210 of the assistance server 200 determinesas well conventional assistance data for the cells served by variousbase stations (step 601). In addition, the processor 210 calculates thesame troposphere model parameter values as described above withreference to FIG. 3 (step 602). However, the center point C_(lon),C_(lat) of a respective model is selected to correspond to the locationof a respective base station or to the center point of a cell served bya respective base station. The processor 210 may determine parametervalues for one or more models for each cell. It is to be understood thatit could also determine parameter values for a common troposphere modelhaving a large fit area for several base stations.

The processor 210 then assembles an assistance message for each basestation (step 601). A message comprises conventional assistance data andin addition parameter values for one or more troposphere models in theformat presented in the table of FIG. 3.

Each base station transmits the provided assistance message as abroadcast message in its cell.

The wireless terminal 100 receives a broadcast message from at least onebase station, and the mobile communication component 110 forwards thecontent of all received broadcast messages to the processor 122. Theprocessor 122 stores the parameter values for all troposphere models inthe database 127 in the memory 126 (step 611).

The GNSS receiver 121 and the processor 122 further use the receivedconventional assistance data for determining the position of thewireless terminal 100 based on received satellite signals (step 612).

The processor 122 then selects parameter values of a suitabletroposphere model from the database 127 as described above (step 613)and calculates the tropospheric delay for all visible satellites asdescribed above (step 614).

The processor 122 then determines the position of the wireless terminal100 anew, taking account of the determined tropospheric delay of eachconsidered satellite signal (step 615).

This process can be repeated with step 613, as long as an applicationneeds updates of the position of the wireless terminal 100.

The content of the troposphere model database 127 in the memory 126 isupdated in parallel, whenever a new broadcast message is received fromthe wireless communication network.

While there have been shown and described and pointed out fundamentalnovel features of the invention as applied to preferred embodimentsthereof, it will be understood that various omissions and substitutionsand changes in the form and details of the devices and methods describedmay be made by those skilled in the art without departing from thespirit of the invention. For example, it is expressly intended that allcombinations of those elements and/or method steps which performsubstantially the same function in substantially the same way to achievethe same results are within the scope of the invention. Moreover, itshould be recognized that structures and/or elements and/or method stepsshown and/or described in connection with any disclosed form orembodiment of the invention may be incorporated in any other disclosedor described or suggested form or embodiment as a general matter ofdesign choice. It is the intention, therefore, to be limited only asindicated by the scope of the claims appended hereto. Furthermore, inthe claims means-plus-function clauses are intended to cover thestructures described herein as performing the recited function and notonly structural equivalents, but also equivalent structures.

1. A method comprising: determining at least one set of values ofparameters, each set of values defining a respective troposphere model;and assembling said at least one determined set of parameter values fortransmission via a wireless communication network to a wireless terminalas assistance data for an assisted satellite based positioning of saidwireless terminal.
 2. The method according to claim 1, wherein saidparameters include at least one parameter for defining a time intervalduring which a troposphere model is valid.
 3. The method according toclaim 2, wherein said at least one parameter for defining a timeinterval comprises a parameter for defining a baseline time and aparameter for defining an interval length.
 4. The method according toclaim 1, wherein said parameters include at least one parameter fordefining an area in which a troposphere model is valid.
 5. The methodaccording to claim 4, wherein said at least one parameter for definingan area comprises parameters for defining a central point of said areaand a parameter for defining a size of said area.
 6. The methodaccording to claim 1, wherein said parameters include at least onezenith delay parameter for defining a tropospheric delay that would beexperienced by a signal transmitted by a satellite and received by saidwireless terminal, when said satellite was located at a zenith of saidwireless terminal.
 7. The method according to claim 6, wherein said atleast one zenith delay parameter include a hydrostatic zenith delayparameter and a non-hydrostatic zenith delay parameter.
 8. The methodaccording to claim 6, wherein said parameters further include at leastone parameter for specifying an adaptation rule for adapting a value ofsaid at least one zenith delay parameter to a current altitude of saidwireless terminal.
 9. The method according to claim 8, wherein said atleast one parameter for defining an adaptation of a value of said atleast one zenith delay parameter to a current altitude h of saidwireless terminal comprises parameters s₀, s₁, e_(h) and e_(w) foradapting a value of a hydrostatic zenith delay parameter Z_(ho)according to the equation:Z _(h)(h)=s ₀ *h+s ₁ *h ² +Z _(h0)*exp(−e _(h) *h) and for adapting avalue of a non-hydrostatic zenith delay parameter Z_(w0) according tothe equation:Z _(w)(h)=Z _(w0)*exp(−e _(w) *h).
 10. The method according to claim 6,wherein said parameters include parameters for at least one mappingfunction, which mapping function maps a value of said zenith delayparameter to a tropospheric delay that is experienced by a signaltransmitted by a satellite and received by said wireless terminal, whensaid satellite is located at a current elevation angle to said wirelessterminal.
 11. The method according to claim 10, wherein said parametersfor at least one mapping function include parameters for a mappingfunction mapping a hydrostatic zenith delay to a hydrostatictropospheric delay and parameters for mapping a non-hydrostatic zenithdelay to a non-hydrostatic tropospheric delay.
 12. The method accordingto claim 10, wherein said parameters further include at least twogradient parameters for defining an azimuthally asymmetric effect of atropospheric delay.
 13. An apparatus comprising a processing component,said processing component being configured to determine at least one setof values of parameters, each set of values defining a respectivetroposphere model; and said processing component being configured toassemble said at least one determined set of parameter values fortransmission via a wireless communication network to a wireless terminalas assistance data for an assisted satellite based positioning of saidwireless terminal.
 14. The apparatus according to claim 13, wherein saidprocessing component is configured to determine for a respectivetroposphere model at least one parameter value defining a time intervalduring which said troposphere model is valid and/or at least oneparameter value defining an area in which said troposphere model isvalid.
 15. The apparatus according to claim 13, wherein said processingcomponent is configured to determine for a respective troposphere modelat least one zenith delay parameter value defining a tropospheric delaythat would be experienced by a signal transmitted by a satellite andreceived by said wireless terminal, when said satellite was located at azenith of said wireless terminal.
 16. The apparatus according to claim13, wherein said processing component is configured to determine for arespective troposphere model at least one parameter value specifying anadaptation rule for adapting a value of said at least one zenith delayparameter to a current altitude of said wireless terminal.
 17. A networkelement of a wireless communication network, said network elementcomprising an apparatus according to claim 13 and a transmittingcomponent configured to transmit at least one set of parameter valuesdetermined and assembled by said apparatus to a wireless terminal asassistance data for an assisted satellite based positioning of saidwireless terminal.
 18. A system comprising an apparatus according toclaim 13 and a wireless terminal configured to receive at least one setof parameter values determined and assembled by said apparatus via awireless communication network.
 19. A software program product in whicha software program code is stored in a readable medium, said softwareprogram code realizing the following when executed by a processor:determining at least one set of values of parameters, each set of valuesdefining a respective troposphere model; and assembling said at leastone determined set of parameter values for transmission via a wirelesscommunication network to a wireless terminal as assistance data for anassisted satellite based positioning of said wireless terminal.
 20. Thesoftware program product according to claim 19, wherein said parametersinclude at least one parameter for defining a time interval during whicha troposphere model is valid and/or at least one parameter for definingan area in which a troposphere model is valid.
 21. The software programproduct according to claim 19, wherein said parameters include at leastone zenith delay parameter for defining a tropospheric delay that wouldbe experienced by a signal transmitted by a satellite and received bysaid wireless terminal, when said satellite was located at a zenith ofsaid wireless terminal.
 22. The software program product according toclaim 21, wherein said parameters further include at least one parameterfor specifying an adaptation rule for adapting a value of said at leastone zenith delay parameter to a current altitude of said wirelessterminal.
 23. An apparatus comprising: means for determining at leastone set of values of parameters, each set of values defining arespective troposphere model; and means for assembling said at least onedetermined set of parameter values for transmission via a wirelesscommunication network to a wireless terminal as assistance data for anassisted satellite based positioning of said wireless terminal.
 24. Amethod comprising: receiving at a wireless terminal via a wirelesscommunication network at least one set of values of parameters, each setof values defining a respective troposphere model; and using anavailable set of values as assistance data for an assisted satellitebased positioning of said wireless terminal.
 25. The method according toclaim 24, further comprising selecting an available set of parametersfor a troposphere model that is valid for a current time and for acurrent location of said wireless terminal as assistance data.
 26. Awireless terminal arrangement comprising: a mobile communicationcomponent configured to receive via a wireless communication network atleast one set of values of parameters, each set of values defining arespective troposphere model; and a positioning component configured touse an available set of values as assistance data for an assistedsatellite based positioning of said wireless terminal arrangement. 27.The wireless terminal arrangement according to claim 26, wherein saidpositioning component is further configured to select an available setof parameters for a troposphere model that is valid for a current timeand for a current location of said wireless terminal arrangement asassistance data.
 28. A software program product in which a softwareprogram code is stored in a readable medium, said software program coderealizing the following when executed by a processor: receiving at awireless terminal via a wireless communication network at least one setof values of parameters, each set of values defining a respectivetroposphere model; and using an available set of values as assistancedata for an assisted satellite based positioning of said wirelessterminal.
 29. An apparatus comprising: means for receiving via awireless communication network at least one set of values of parameters,each set of values defining a respective troposphere model; and meansfor using an available set of values as assistance data for an assistedsatellite based positioning of a wireless terminal.